Time course of the EPSPs. *A*: presynaptic action potential (AP, thin black line) and mean evoked EPSP (thick black line) in a PC–PC pair. The EPSP amplitudes were normalized to 1 and the EPSPs were fit by a difference of 2 exponentials (gray dashed line) to obtain time constants for the rise (τ_{r}) and decay (τ_{d}). Scale bars: horizontal: 20 ms (AP and EPSP); vertical: 40 mV (AP) and 0.5 normalized units (EPSP). The latency was defined as the difference between the onset of the AP (vertical, dashed line) and the onset of the EPSP (arrowhead). *B*: distribution of latencies over all pairs. *C*: latency vs. distance between somata. *D*: the distribution of EPSP peak times (difference between the onset of the EPSP and the maximum) for all pairs. *E*: the distribution of EPSP rise constants (τ_{r}). *F*: the synaptic decay constant (τ_{d}) vs. postnatal day age for individual pairs (open circles) and the mean ± SD for each age (filled circles). *G*: the distributions of EPSP decay constants (τ_{d}) for P10–P14 (open bars), P15–P18 (gray bars), and P19–P29 (black bars). τ_{d} decreased significantly with age (***P* < 0.01). *H*: the mean τ_{d} plotted against the mean resting membrane potential (*V*_{m}, *left*), membrane time constant (τ_{m}, *middle*), and mean input resistance (*R*_{n}, *right*) for each postnatal day. τ_{d} was positively correlated with *V*_{m} (*R*: 0.92), τ_{m} (*R*: 0.83), and *R*_{n} (*R*: 0.93, black fit lines). Error bars ± SD. *I*: voltage- and current-clamp recordings of evoked excitatory postsynaptic currents (EPSCs) and EPSPs in the same PC–PC pair. *Left*: EPSCs recorded at different holding potentials (HPs) from −80 to −30 mV. *Right, top*: the same EPSCs shown *left*, normalized to the peak amplitude, for 3 holding potentials: −80 mV (black), −60 mV (gray), and −30 mV (dashed). *Right, bottom*: normalized EPSPs recorded in the same pair at −60 mV (gray) and −80 mV (black). The resting membrane potential of the postsynaptic cell was −68 mV. Scale bars: horizontal: 20 ms, vertical: 100 pA.

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